Experiments dealing with the effect of calcium concentration on the magnitude of the C1 component of the Limulus ventral eye receptor potential will be continued, including the measurement of reversal potential of C1 and the effect of Ca2ion blockers. Upon completion of a programmable photostimulator, a series of experiments will be performed to attempt to fractionate the latent period of the receptor potential, as suggested by the model developed by Selig Hecht more than a half century ago.